It is well known that on a global scale, at least 100 times more water is stored in aquifers than in rivers and lakes combined (Shiklomanov and Rodda, 2003). This makes groundwater systems comparatively resilient to short-term, seasonal and even longer-term shortage of rainfall. Climate change is, nevertheless, likely to impact upon the quantity and quality of groundwater resources, and alluvial aquifers in arid regions rank as the most vulnerable to climate change. Groundwater is often, especially in arid regions, the most reliable source of water – if not the only one – for domestic water supply and irrigation of crops. This case study in Yemen explores to what extent groundwater in alluvial aquifers in arid regions may be affected by climate change during the 21st century.